1. Technical Field
The invention relates to sensors. In particular, the invention relates to sensors that employ optical spectroscopy to analyze a material.
2. Description of Related Art
Spectroscopy is a powerful means for analyzing, characterizing and even identifying a substance or material using one or both of an absorption spectrum and an emission spectrum that results when the material is illuminated by a form of electromagnetic radiation. For example, optical spectroscopy generally involves illuminating the material with an optical signal (e.g., light) and one or both of observing and measuring a spectrum of a response signal produced by an interaction between the optical signal and the material. For example, in optical absorption spectroscopy, an absorption spectrum is compared to a spectrum of the optical signal to determine a spectral ‘fingerprint’ of the material. In general, the spectral fingerprint is characteristic of the particular material or its constituent parts facilitating identification of the material. Another exemplary form of optical emission spectroscopy is based on Raman-scattering. In Raman-scattering optical spectroscopy, an emission spectrum or spectral components thereof produced by an interaction between the material and the optical signal are associated with particular characteristics (e.g., chemical make-up) of the material. These spectral components contained in a response signal facilitate determination of the material characteristics. An intensity of the Raman-scattering may be significantly enhanced by using a Raman-active material (e.g., Raman-active surface). For example, a Raman-active surface may be employed in surface enhanced Raman-scattering (SERS) optical spectroscopy.
Optical spectroscopy performed in hollow core optical waveguides (e.g., hollow core optical fibers) has generated considerable interest. In particular, using hollow core optical waveguides has the potential of solving some problems and deficiencies that may arise with various forms of optical spectroscopy. For example, the hollow core optical waveguide provides a means for containing the material and one or both of guiding the optical signal to the material and guiding a response signal away from the material. A length over which the material and the optical signal interact (i.e., interaction length) can be made essentially arbitrarily long to increase a signal strength of the response signal, such that detection thereof is improved. Moreover, detection may be further enhanced by using the optical waveguide to preferentially guide the response signal to the detector. However, performing optical spectroscopy in a hollow core optical waveguide presents its own set of problems including, but not limited to, integrating the hollow core optical waveguide with other system components and providing means for introducing enhancement factors (e.g., a Raman-active surface) within the confines of the hollow core optical waveguide.